The invention relates to load cells, and in particular to load cells for use in sensing weight and position of a seated occupant in a motor vehicle for deployment of safety devices, such as air bags.
Since the development of the air bag and its inclusion in automobiles, a problem has existed with the relative deployment force used for various individuals. Air bags have been a requirement on new vehicles since 1992. Air bags are made to arrest the forward momentum of the driver or passenger in the event of a collision. If one designs a universal air bag for all passengers, then it must have sufficient force to stop the largest of the expected passengers. Smaller passengers have less momentum, and so do not require the same momentum change as the larger passenger. In addition, smaller passengers are shorter, and sit closer to the dashboard, and therefore experience more of the bag""s explosive force than a average adult male, sitting further back. As a result of the current air bag deployment force, there have been a number of injuries and fatalities associated with air bag deployment. As of mid 1998, 105 deaths have been attributed to the deployment of air bags with a small adult or a young child when no air bag deployment would not have resulted in any injury to the occupant.
This situation has caused NHTSA, the National Highway Traffic Safety Administration, a branch of the U.S. Department of Transportation, to propose rules which will change the criteria for air bag activation, as well as the deployment force, in order to protect such small occupants. In addition to these requirements, the NHTSA has also identified xe2x80x9cout of positionxe2x80x9d occupants as a source of concern. Thus, a system must be able to modulate or reduce the air bag deployment force if the occupant is in a position such that its deployment would result in injury, even if that occupant is a full size adult.
There are several methods which can sense the presence and weight of an occupant. In U.S. Pat. No. 5,573,269, Gentry et al. teach an apparatus which uses weight measurements, using a sensor, in an automobile seat as an input to a controller which operates air bags. This sensor, described in U.S. Pat. No. 5,494,311, is a thin structure that resides in the bottom seat cushion. As is recognized by Gentry, much of the occupant""s weight is also directed into the seat back, thereby bypassing the weight sensing pad and traveling directly through the seat structure to the chassis of the vehicle. An incline sensor, which measures the tilt of the back of the seat is also provided to compensate for this effect.
There are two problems with this system. First it assumes that the weight can be determined only by the pressure on the seat cushion bottom and by the angle of the seat. That is not always the case. Consider an occupant who puts horizontal pressure on the floorboard in front of the seat. This increases the force on the back with a resulting decrease on the bottom cushion. At some point this pressure can be great enough that nearly all of the occupants weight is on the back cushion. This problem is also present in U.S. Pat. No. 5,474,327. In this device a set of pressure sensitive pads is placed beneath the surface of the seat cushion. While this device is adequate for the detection of a child seat, it does not give adequate information for small adults and out of position occupants.
Blackburn et al. teaches in U.S. Pat. No. 5,494,311 a system where pads are placed in both the lower and rear seat cushion. This gives a better weight measurement under all conditions, the obvious downside is the cost.
One of the problems of prior systems is that they cannot read negative weight, i.e. when forces are present that would cause the force on the seat support to go negative. This can occur when the occupant places force, via his feet, on the front of the passenger compartment.
Yet another difficulty is that since the pressure is sensed on the seat, the seat belt tension adds to the reading. A 40 pound car seat could then, with sufficient tension on the seat belt, put 200 pounds of force on the seat surface, causing a false reading.
There is a need for a sensor capable of accurate determinations of weight in an automotive application. More generally, accurate load cells capable of accurate weight determinations of off-center loads is desirable.
A method for making a load cell in accordance with the present invention includes providing a member having a curved portion and opposed segments. The curved member is so configured that opposing forces applied to the opposed segments produces torsion in the curved portion. Strain gauges are formed on the opposed segments proximate the curved portion to sense the torsional stress.